Plasma arc cutting device having metal deposition removal function and method for removing deposition from nozzle of the plasma arc cutting device

ABSTRACT

A plasma arc workpiece cutting device having means for removing a deposited material deposited onto a nozzle of a plasma torch. During plasma arc cutting operation, molten spatters may be directed toward the plasma torch and may be adhered onto the nozzle. The deposited material is removed by way for removing the deposited material. At every proper intervals, the plasma torch is moved toward the removing structure, and the deposited material is brought into sliding contact therewith in order to frictionally remove the deposited material from the nozzle. A control system is provided for moving the plasma torch at every proper timings so as to perform the deposited material removing operation.

BACKGROUND OF THE INVENTION

The present invention relates to a plasma arc cutting device havingmeans for removing a deposition from a plasma arc nozzle, and to amethod for removing the deposition from the nozzle.

A plasma arc cutting device is well known in the art in which a metallicor non-metallic workpiece is locally melted and cut by a heat energy ofthe plasma arc passing through a plasma arc nozzle. In a conventionalplasma arc cutting method, a through hole is formed in a plate likeworkpiece as a starting hole by the ejection cf the plasma arc on theworkpiece, and thereafter, linear cutting starting from the startinghole is carried out by the plasma arc.

As shown in FIG. 1(a) , the plasma arc cutting device is provided with anozzle 22, through which a main plasma arc 1 is passed. Within thenozzle 22, an electrode 21 is provided for generating the plasma arc 1.Apparently, the workpiece W and the electrode 21 are connected to apower source for generating the main plasma arc 1 between a tip end ofthe electrode 21 and the surface of the workpiece W. An annular space isprovided between the electrode 21 and the nozzle 22.

For forming the starting hole in the workpiece W, material of theworkpiece W melted by the plasma arc 1 is splashed as spatters 2 sincethe melted material cannot be directed to a proper location. Parts ofthe spatters may be reflected on the surface of the workpiece W, and aredirected upwardly to the nozzle 22. The reflected molten material may bedeposited on the nozzle 22, and the deposition may be largely grown asshown in FIG. 2(b). If this deposition is grown to a certain mass,another arc 4 between the electrode 1 and the nozzle 22 and arc 5between the deposition 3 and the workpiece W are generated in additionto the main arc plasma 1. Such plurality of arcs may degrade theworkpiece cutting efficiency and may deteriorate the cutting contours inthe workpiece.

If the deposition 3 is further grown to reach the workpiece W, the mainplasma arc 1 may disappear, and large internal plasma arc 6 may begenerated between the electrode 21 and the nozzle 22 as shown in FIG.1(c) . This internal plasma arc 6 may deteriorate the nozzle 22.

According to the conventional plasma arc cutting device, however, noparticular attention is drawn to such a deposition 3 onto the nozzle 22.An operator is, therefore, obliged to manually remove the deposition byusing a file, etc. Such manual removal of the deposited material 3 maybe troublesome. Particularly, if the plasma arc cutting device isoperated under Numerical Control (NC) in which the device isautomatically operated, the working efficiency may be extremely loweredif monitoring the state of the deposited material and if intermittentlybreaking off the cutting operation for conducting the manual operationin order to manually remove the deposited material.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to overcome theabove described drawbacks and disadvantages and to provide an improvedapparatus and method for automatically removing the materials depositedon the nozzle.

Another object of the invention is to provide an improved plasma arccutting device and method for removing the deposition in whichinadvertent surplus arcing is avoidable by properly removing thematerial deposited on the nozzle.

These and other object of the present invention will be attained byproviding a plasma arc cutting device for cutting a workpiece with aplasma arc comprising: a table for mounting the workpiece thereon, aplasma torch comprising a gas supply nozzle and an electrode forgenerating a plasma arc jet toward the workpiece, moving means forrelatively moving the plasma torch with respect to the table, a plasmaarc power supply unit connected to the plasma torch for supplyingelectrical current thereto to thereby provide the plasma arc between theelectrode and the workpiece, means for removing a material deposited onthe nozzle therefrom, the deposited material removing means beingprovided on the table, the plasma torch being movable toward and awayfrom the removing means, and control means for controlling movement ofthe moving means for controlling relative position between the plasmatorch and the workpiece, the control means having means for executingmovement of the moving means at an interval so as to position the plasmatorch in confrontation with the removing means to thereby remove thedeposited material from the nozzle.

In another aspect, in accordance with the present invention there isprovided a method for removing a material deposited onto a nozzle of aplasma torch in a plasma arc cutting device, the device including atable for mounting a workpiece thereon, the plasma torch comprising agas supply nozzle and an electrode for generating a plasma arc jettoward the workpiece, moving means for relatively moving the plasmatorch with respect to the table, and a plasma arc power supply unitconnected to the plasma torch for supplying electrical current theretoto thereby provide the plasma arc between the electrode and theworkpiece, the method comprising the steps of: providing a means on thetable for removing the deposited material from the nozzle;

intermittently moving the plasma torch toward the deposition removingmeans, and sliding the deposited material with respect to the removingmeans so as to frictionally remove the deposited material from thenozzle.

In accordance with the present invention, the deposited material can beremoved at every proper intervals even during the exact workpiececutting operation. Therefore, the deposited material can be partly orentirely removed from the nozzle before the deposition largely grows.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings;

FIGS. 1(a) through (c) are schematic cross-sectional views fordescription of material deposition into a nozzle in a conventionalplasma arc cutting device;

FIG. 2 is a perspective view showing a plasma arc cutting deviceaccording to one embodiment of the present invention;

FIG. 3 is a block diagram showing electrical circuit for automaticcontrol to the plasma arc cutting device of the present invention;

FIGS. 4 to 6 and FIG. 8 are perspective views showing examples offriction units for removing the deposited material, the unit beingusable in the embodiment of this invention;

FIG. 7 is a plan view showing another example of a friction unit forremoving the deposited material;

FIG. 9 is a front view showing the example of the friction unit shown inFIG. 8; and

FIG. 10 is a flowchart for description of sequential control to removethe deposited material according to one embodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A plasma arc cutting device according to one embodiment of thisinvention will be described below with reference to accompanyingdrawings.

In FIG. 2, the plasma arc cutting device has a gate or arch shapedmovable frame 12 movable in one horizontal direction (Y direction)relative to a stationary table 11 on which a workpiece W is mounted. AY-axis motor 13 is supported on the table 11 for moving the movableframe 12 in Y direction. On the movable frame 12, a horizontallyextending guide member 18 is fixed, and a saddle member 14 is movablysupported on the guide member 18. The saddle member 14 is movable in ahorizontal direction (X direction) perpendicular to the Y direction. AX-axis motor 15 is fixedly supported on the movable frame 12 for movingthe saddle member 14 along the guide 18. On the saddle member 14, a head16 is movably supported. The head 16 is vertically movable (Z direction)by a Z-axis motor 17 fixedly supported on an upper end face of thesaddle member 14.

The head 16 has a tip end portion provided with a plasma torch 20 whichis positioned in confrontation with the workpiece W mounted on the table11. The plasma torch 20 is adapted for ejecting a plasma arc which areto be directed on the workpiece W. Further, a friction unit 30 having afriction surface 31 is fixed on the table 11. A tip end portion of theplasma torch 20 can be brought into sliding contact with the frictionsurface 31.

As shown in FIG. 3, the Y-axis motor 13, the X-axis motor 15, the Z-axismotor 17 and a torch 20 are connected to a control unit 50. Further, aCRT display 51 and a keyboard 52 are also connected to the control unit50. The control unit 50 includes a computer having a central processingunit (CPU) 53, a ROM 54 and a RAM 55. By the manipulation to thekeyboard 52, the X-axis motor 15, the Y-axis motor 13 and the Z-axismotor 17 are controlled, and the cutting operation is observed by theCRT display 51.

The CPU 53 is connected to a plasma arc power supply unit 56 and a gassupply unit 57 for their controls. The plasma arc power supply unit 56is connected to a commercial power source, and generates a directcurrent or pulsating current in accordance with a command from the CPU53, which current is supplied to an electrode 21 of a plasma torch 20.The gas supply unit 57 is adapted for supplying a gas such as oxygen tothe plasma torch 20.

The plasma torch 20 has the nozzle 22 formed of a copper, and theelectrode 21 deposed within an internal space of the nozzle 22. Thenozzle 22 defines an ejection opening 23 through which the oxygen gasfrom the gas supply unit 57 is ejected toward the workpiece W. Apositive terminal of the plasma arc power supply unit 56 is connected tothe workpiece W whereas a negative terminal of the unit 56 is connectedto the electrode 21. The plasma arc generated between the electrode 21and the workpiece W is grown to a plasma jet by the gas blown throughthe nozzle 22, and the plasma jet is applied to the workpiece W.

FIG. 4 shows one example of the friction unit 30. The unit includes aflat grinding stone 31 as the friction member fixedly secured to a base30'. The friction unit 30 is formed with bores 33 through which boltsextend to fix the friction unit 30 to the table 11. Instead of thegrinding stone, a sand paper (not shown) can merely be attached on thetable 11.

FIG. 5 shows another example of a friction unit. The friction unitincludes vertically orienting wire brush 34 implanted on the base 30'.

FIG. 6 shows still another example of a friction unit 35. The unit is ofa block configuration and is made of steel. The unit 35 is formed with alinear groove 36 whose cross-sectional contour is identical with across-sectional contour of the nozzle 22 provided at the tip end portionof the plasma torch 20. When the nozzle 22 is aligned with the groove36, and runs therealong, any material deposited on the nozzle 22 isremovable. In this connection, the groove 36 serves as a frictionsurface with which the nozzle 22 is in sliding contact.

FIG. 7 shows still another example of a block like friction unit 39.This unit is formed with a groove 38 having a cross-sectionalconfiguration the same as that of the groove 36 shown in FIG. 6.However, the groove 38 includes a linear section and a circular section37 tangentially contiguous with the linear section. By moving the nozzle22 along the full circle of the circular groove 37, material depositedon an outer peripheral surface of the nozzle 22 can be removedtherefrom.

FIGS. 8 and 9 show still another example of a friction unit 40. The unit40 includes a base 41 and a plurality of upstanding plates 42, 43 and 44those extending from the base 41. The plates 42, 43 and 44 are formedwith notches 45, 46 and 47, respectively each having configurationanalogous to the cross-sectional configuration of the nozzle 22. Here,as best shown in FIG. 9, the first notch 45 which is adapted to firstallow the nozzle to pass therethrough has the largest configuration, sothat the largest gap is provided between the first notch 45 and thenozzle 22. The gap is gradually reduced, so that the last notch 47 has aconfiguration to permit the nozzle 22 to slidingly move with respectthereto. With this structure, the largely bulged or protruded depositedmaterial is chipped off from the nozzle 22, when the latter is passedthrough the first notch 45, and the deposited material is graduallyremoved orderly when the nozzle 22 is passed through the second and thelast notches 46 and 47.

An operational routine controlled by the control unit 50 will next bedescribed with reference to FIG. 10. In step 100, a processing isstarted. At this time, initial setting, working data read-out etc. areexecuted. Then, in Step 111, the plasma arc power supply unit 56 and thegas supply unit 57 are operated for forming the starting hole i,e,piercing is carried out. Upon completion of the piercing, the routineproceeds into Step 112 where cutting to the workpiece W is carried out.In this cutting process, the X-axis motor 15 and the Y-axis motor 13 aredriven in accordance with operational program stored in the RAM 55, sothat the workpiece W is cut into a desired configuration.

When the sequential cutting operation is completed, the routine proceedsinto Step 113 in which investigated is the already executed numbers offormations of the starting holes. If the numbers reaches a predeterminednumbers such as seven times, the routine proceeds into Step 115, and ifthe numbers does not reaches the predetermined numbers, the routineproceeds into Step 114. In Step 114, judgment is made as to whether ornot the instruction indicative of the removal of the deposited materialfrom the nozzle 22 is issued from the keyboard 52. If the judgment fallsYes, i.e., the instruction has already been issued, the routine proceedsinto Step 115, and if the judgment falls No, the routine proceeds intoStep 116.

In the Step 115, the CUP 53 sends an output signal to the motors 13, 15and 17 so as to move the plasma torch 20 away from the workpiece W andto direct the torch 20 toward a position immediately above the frictionunit 30 in order to carry out the removal of the material deposited onthe nozzle 22 therefrom. The head 16 is then moved downwardly byoperating the Z-axis motor 17, and thereafter, the plasma torch 20 isfurther moved by operating the X-axis motor 15 and the Y-axis motor 13so that the nozzle 22 can be brought into sliding contact with or intoclose access to the friction surface 31 of the friction unit 30. As aresult, the material 3 deposited on the nozzle 22 can be brought intosliding contact with the friction surface 31 so as to frictionallyremove the deposited material from the nozzle 22. When the removal workof the deposited material is completed, the routine proceeds into Step116 where judgment is made as to whether or not the workpiece cuttingoperation is finished. If No, the routine returns back to Step 111 andthe above described proceedings are again repeated. On the other hand,if the cutting operation with respect to the workpiece W is alreadyfinished, the routine goes to Step 117 to end the process.

In the above described process, the control unit 50 and the keyboard 52serve as signal generating means for generating deposited materialremoving signal in association with the executed Step 113 and Step 114in accordance with the completion of the predetermined working, such ascompletion of the predetermined times of formations of the startingholes, and in accordance with the manipulation to the keyboard 52.Further, the control unit 50 serves as instruction means for instructinga start of the deposited material removing operation from the nozzle 22in response to the deposited material removing signal generated at theStep 113 or Step 114. By means of the instruction means, the X-, Y- andZ-axis motors 15, 13 and 17 are controlledly rotated so that the nozzle22 provided at the tip end portion of the plasma torch 20 is broughtinto sliding contact with or brought into a position immediatelyadjacent to the friction surface 31, 34, 36, 37, 38, 45, 46 or 47 of thefriction unit 30, 34, 35, 39 or 40, to thereby remove the depositedmaterial from the nozzle 22.

In the above described embodiment, the numbers of formations of thestarting holes is counted, and if the counted numbers reaches thepredetermined numbers, the deposited material removing signal isgenerated, i.e, the deposited material removing operation is started.However, the deposited material removing operation can also be startedby the other factors. For example, cumulation is made with respect tototal period for the exact formation of the starting holes and cuttingoperation, and the exact accumulated period is compared with apredetermined period for the judgment of the start of the depositedmaterial removal work. Alternatively, the working distance iscumulatively stored into RAM and the cumulated distance is compared witha predetermined distance for the judgment of the start. Furtheralternatively, at every replacement of the workpiece W, the replacementtimes is counted, for example, the operation start signals are counted,and if the counted numbers reaches a predetermined numbers, the routinegoes into Step 115. In still further alternative, a predeterminedvoltage is set in the RAM, and if an exact voltage between the nozzle 22and the pate like workpiece W is lowered to the predetermined voltage,the routine can goes into the Step 115.

As described above, in the present invention, the material depositedonto the tip end of the plasma torch can be automatically removedtherefrom. Therefore, generation of double arcing is avoidable.Accordingly it is unnecessary for an operator to continuously monitorthe exact piercing or cutting state to the workpiece, and stabilizedautomatic plasma arc cutting operation is attainable without destructionof the nozzle or the plasma torch.

While the invention has been described in detail and with reference tospecific embodiment thereof, it would be apparent to those skilled inthe art that various changes and modifications may be made thereinwithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A plasma ac cutting device for cutting anelectrically conductive workpiece with a plasma arc comprising:a tablefor mounting the workpiece thereon; a plasma torch comprising a gassupply nozzle and an electrode for generating a plasma arc jet towardthe workpiece, the plasma arc contacting the workpiece; moving means forrelatively moving the plasma torch with respect to the table; a plasmaarc power supply unit connected to the plasma torch for supplyingelectrical current thereto to thereby provide the plasma arc between theelectrode and the workpiece; means for removing a material deposited onthe nozzle therefrom, the deposited material removing means beingstationary and fixed to the table, the plasma torch being movable towardand away from the removing means, and control means for controllingmovement of the moving means for controlling relative position betweenthe plasma torch and the workpiece, the control means having means forexecuting movement of the moving means at a predetermined interval so asto position the plasma torch in confrontation with the removing means tothereby remove the deposited material from the nozzle and timing meansfor determining the interval.
 2. The plasma arc cutting device asclaimed in claim 1, wherein the execution means comprises:a signalgenerating means for generating a signal indicative of a removal of thedeposited material in response to the interval determined by the timingmeans; instruction means for instructing the start of the depositedmaterial removing operation in response to the signal so as to move themoving means to permit the plasma torch to move toward the removingmeans.
 3. The plasma arc cutting device as claimed in claim 1, whereinthe removing means comprises a base fixedly secured to the table, and aflat grinding stone secured to the base, the grinding stone serving as afriction surface, the deposited material being in sliding contact withthe friction surface upon movement of the moving means.
 4. The plasmaarc cutting device as claimed in claim 2,wherein the removing meanscomprises a base fixedly secured to the table, and a flat grinding stonesecured to the base, the grinding stone serving as a friction surface,the deposited material being in sliding contact with the frictionsurface upon movement of the moving means.
 5. The plasma arc cuttingdevice as claimed in claim 1, wherein the removing means comprises afriction block fixed to the table, the friction block being formed witha linear groove having a cross-sectional configuration identical with across-sectional configuration of the plasma torch, a surface of thegroove serving as a friction surface and the deposited material being insliding contact with the friction surface upon movement of the movingmeans.
 6. The plasma arc cutting device as claimed in claim 2, whereinthe removing means comprises a friction block fixed to the table, thefriction block being formed with a linear groove having across-sectional configuration identical with a cross-sectionalconfiguration of the plasma torch, a surface of the groove serving as afriction surface and the deposited material being in sliding contactwith the friction surface upon movement of the moving means.
 7. Theplasma arc cutting device as claimed in claim 1, wherein the removingmeans comprises a friction block fixed to the table, the friction blockbeing formed with a linear groove and a circular groove tangentiallyprovided relative to the linear groove, surface of the linear groove andthe circular groove serving as a friction surface and the depositedmaterial being in sliding contact with the friction surface uponmovement of the moving means.
 8. The plasma arc cutting device asclaimed in claim 2, wherein the removing means comprises a frictionblock fixed to the table, the friction block being formed with a lineargroove and a circular groove tangentially provided relative to thelinear groove, surface of the linear groove and the circular grooveserving as a friction surface and the deposited material being insliding contact with the friction surface upon movement of the movingmeans.
 9. The plasma arc cutting device as claimed in claim 1, whereinthe removing means comprises a base fixedly secured to the table, and aplurality of plates vertically extending from the base and arrayedlinearly, each of the plurality of plates being formed with a notchhaving a configuration analogous to a cross-sectional configuration ofthe plasma torch, a size of a precedent notch being greater than asubsequent notch, so that a gap between a notch and the plasma torch isgradually reduced toward the subsequent notch in accordance with themovement of the plasma torch over the plurality of plates, faces of thenotches serving as friction surfaces.
 10. The plasma arc cutting deviceas claimed in claim 2, wherein the removing means comprises a basefixedly secured to the table, and a plurality of plates verticallyextending from the base and arrayed linearly, each of the plurality ofplates being formed with a notch having a configuration analogous to across-sectional configuration of the plasma torch, a size of a precedentnotch being greater than a subsequent notch, so that a gap between anotch and the plasma torch is gradually reduced toward the subsequentnotch in accordance with the movement of the plasma torch over theplurality of plates, faces of the notches serving as friction surfaces.11. The plasma arc cutting device as claimed in claim 1, furthercomprising manual timing determination means for invalidating theinterval determined by the timing means and for outputting a signal tothe control means in order to execute a removal of the depositedmaterial on a basis of a manually determined timing.
 12. A method forremoving a material deposited onto a nozzle of a plasma torch in aplasma arc cutting device, the device including a table for mounting anelectrically conductive workpiece thereon, the plasma torch comprising agas supply nozzle and an electrode for generating a plasma arc jettoward the workpiece, the jet contacting the workpiece, moving means forrelatively moving the plasma torch with respect to the table, and aplasma arc power supply unit connected to the plasma torch for supplyingelectrical current thereto to thereby provide the plasma arc between theelectrode and the workpiece, the method comprising the stepsof:providing a stationary means fixed on the table for removing thedeposited material from the nozzle; determining an interval of movementof the plasma torch; intermittently moving the plasma torch toward thedeposition removing means based on the determined interval of movement;and sliding the deposited material with respect to the removing means soas to frictionally remove the deposited material from the nozzle. 13.The method for removing the deposited material from the nozzle of theplasma torch as claimed in claim 12 wherein the interval determinationstep comprises the steps of:generating a signal indicative of a removalof the deposited material in response to a given result; and instructinga start of the deposited material removing operation in response to thesignal so as to move the moving means to permit the plasma torch to moverelative to the removing means.
 14. The method as claimed in claim 13,wherein the given result comprises a result of comparison of apredetermined stored numbers with an actual numbers of formations ofstarting holes to be formed in workpieces, the signal being generatedwhen the actual numbers reaches the predetermined stored numbers. 15.The method as claimed in claim 13, wherein the given result comprises aresult of comparison of a predetermined stored period with actualcumulative period for performing starting hole forming operation andcutting operation with respect to a workpiece, the signal beinggenerated when the actual cumulative period reaches the predeterminedstored period.
 16. The method as claimed in claim 13, wherein the givenresult comprises a result of comparison of a predetermined storeddistance with actual working distance for cutting a workpiece, thesignal being generated when the actual working distance reaches thepredetermines stored distance.
 17. The method as claimed in claim 13,wherein the given result comprises a result of comparison of apredetermined stored numbers with actual numbers of replacingworkpieces, the signal being generated when the actual numbers reachesthe predetermined stored numbers.
 18. The method as claimed in claim 13,wherein the given result comprises a result of comparison of apredetermined stored voltage with actual voltage between the nozzle andthe workpiece, the signal being generated when the actual voltage isreduced to the predetermined stored voltage.
 19. A plasma arc cuttingdevice for cutting an electrically conductive workpiece with a plasmaarc comprising:a table for mounting the workpiece thereon; a plasmatorch comprising a gas supply nozzle and an electrode for generating aplasma arc jet toward the workpiece, the plasma arc contacting theworkpiece; moving means for relatively moving the plasma torch withrespect to the table; a plasma arc power supply unit connected to theplasma torch for supplying electrical current thereto to thereby providethe plasma arc between the electrode and the workpiece; means forremoving a material deposited on the nozzle therefrom, the depositedmaterial removing means being provided on the table, the plasma torchbeing movable toward and away from the removing means, the removingmeans comprising a base fixedly secured to the table and a flat grindingstone secured to the base, the grinding stone serving as a frictionsurface and the deposited material slidingly contacting the frictionsurface upon movement of the moving means; and control means forcontrolling movement of the moving means for controlling relativeposition between the plasma torch and the workpiece, the control meanshaving means for executing movement of the moving means at an intervalso as to position the plasma torch in confrontation with the removingmeans to thereby remove the deposited material from the nozzle.